i was using the search function (found no code that was doing the job), and now i tried your suggestion:

[Desktop Entry]
Type=Application
Name=gainroot
Icon=terminal
Exec=/bin/busybox sh -c 'echo /home/user/MyDocs/Scripts/gainroot.sh | sudo gainroot'

But even this won't work...the system is doing something but no xterm is openening. Btw: your zip-file doesn't contain any .desktop file.